Molecular beam source



Oct. 9, 1962 .1. GEORGE 3,058,023

MOLECULAR BEAM SOURCE Filed March 9, 1960 INVENTOR. JAMES GEORGEATTORNEYS United States 3,058,023 MOLECULAR BEAM SOURCE James George,Swampscott, Mass, assignor to National Company, Inc., Malden, Mass, acorporation of Massachusetts Filed Mar. 9, 1960, Ser. No. 13,909 9Claims. (Cl. 313-231) This invention relates to a molecular beam sourcefor supplying a molecular beam of cesium or the like from a liquidreservoir under high vacuum conditions. More particularly, it relates toa molecularbeam source which evaporates liquid cesium and forms theresulting vapor into a beam for use in a molecular beam frequencystandard. The source is capable of operation in all orientations andunder conditions of severe vibration and inertial forces with negligibleleakage of the liquid from which the gaseous molecular beam is evolved.

The present invention may be used in various types of molecular beamapparatus. -It is of particular utility in a molecular beam frequencystandard which utilizes the interaction of microwave energy of aparticular wave length with a cesium beam in a frequency-determiningarrangement. As used herein, the words molecule and molecular refer toboth atoms of a single element and molecules 'of a compound; hence, thecesium beam is referred to as a molecular beam, although it is composedof cesium atoms. An instrument of this type is fully described in thecopending application of Jerrold R. Zacharias, et al. for Molecular BeamApparatus, Serial No. 693,104, filed October 22, 1957 and assigned tothe assignee of the present application, now Patent No.

cules into a beam for passage through the beam tube.

iFrequency standards of the above type may be used in moving vehiclessuch as airplanes or missiles, where they are subject to severe inertialforces including substantial vibration; they also may be subject tocomplete change of orientation. Motion of this type has, in priorconstructions, caused spillage of liquid cesium from the beam source.Since the only supply of heat for evaporation of the charge is generallyintimately associated with the source, the cesium lost therefrom remainsin liquid form or reverts to the solid state and is therefore of nofurther use in the frequency standard. The time during which thestandard may be operated before being recharged with cesium isconsequently materially reduced. The reliability of the apparatus isalso adversely affected by the fact that the spilled cesium may lie inportions of the beam tube and associated components of the frequencystandard in such manner as to cause serious malfunctions.

Accordingly, it is a principal object of my invention to provide animproved molecular beam source adapted to supply a beam of gaseouscesium or the like in 1a molecular beam instrument, particularly amolecular beam frequency standard.

A further object of the invention is to provide a reservoir designed forincorporation in a beam source of the character described which isadapted to retain a charge of liquid cesium therein under conditions ofintense inertial forces and changes or orientation.

Another object of the present invention is to provide ice an improvedbeam source of the above character adapted to prevent leakage of liquidcesium into the frequency standard.

Since the frequency standard may be operated in moving vehicles such asairplanes or the like, the source should be small in size and capable oflightweight construction. Simplicity of design is also a feature of myinvention, since it results in low cost fabrication.

Other objects and features of my invention will in part be obvious andwill in part appear hereinafter.

My invention accordingly comprises the features of construction,combination of elements, and arrangement of parts which will beexemplified in the construction hereinafter set forth, and the scope ofthe invention will be indicated in the claims.

For a fuller understanding of the nature and objects of the invention,reference should be had to the following detailed description taken inconnection with the accompanying drawing in which:

FIGURE 1 is a sectional view of a molecular beam source made accordingto my invention, and

FIGURE 2 is a sectional view taken along line 2-2 of FIGURE 1 showing indetail a collimator structure which may be used in the source of FIGURE1.

In general, a molecular beam source embodying the features of myinvention includes a reservoir containing a charge of liquid cesium anda collimator which forms the gaseous molecules evaporated from thecharge inito a beam. A barrier interposed between the reservoir and thecollimator permits the passage of the gaseous molecules while preventingthe escape of liquid from the reservoir. A heating element maintains theliquid cesium at a temperature which provides a gas pressure behind thecollimator consonant with the desired flow rate from the source.

in accordance with my invention, the reservoir comprises a mass ofpacked fibers such as deoxidized steel wool, capable of being wet by thecesium charge. The charge is completely adsorbed by the wool. That is,the liquid cesium spreads out along the fibers to form a thin filmthereon. This film, which may be only a few atoms thick, will ordinarilynot be dislodged by even the severest vibrations, and therefore thecesium generally issues from the reservoir only in gaseous form throughevaporation of the liquid charge.

The barrier may also be of a porous fibrous material such as steel wool.However, the surfaces of the fibers should not be wetted by the cesiumcharge. The nonwetting characteristic may be obtained by formingchromium oxide on these surfaces. Thus, assuming that the material ofthe barrier is tightly packed so that the interstices between the fibersare sufliciently small, liquid cesium will be prevented from passingthrough the barrier in much the same manner as a liquid is preventedfrom passing through a capillary tube whose inner surface is not wettedby it. On the other hand, the passage of gaseous molecules is notaffected by the non-wetting characteristics of the barrier, andtherefore cesium vapors from the reservoir freely diffuse through thebarrier toward the collimator.

It will be understood that while the apparatus to be described below ingreater detail is specifically designed to be incorporated in amolecular beam frequency standard using an atomic resonance of thecesium atom as a reference frequency, the present invention is notlimited to use with this particular element. Thus, my beam source may,in many cases, be used with frequency standards utilizing thecharacteristics of others of the alkaline metals or other materialshaving the proper molecular characteristics for use in molecular beamequipmerits.

As seen in FIGURE 1, my molecular beam source may be housed in a tube 10closed at one end (the left end in FIGURE 1) by a cap 12 and providedwith a flange 14 at the other end to facilitate attachment to theapparatus which is to use the beam. The tube 10 contains a cesiumreservoir 16, a barrier 18 adjacent to the reservoir and a collimatorgenerally indicated at 20 spaced from the barrier 18 by a chamber 22. Aheating coil 24 is formed around the tube 10 over the collimator 20. Theentire unit is encased in a heat insulation covering 26.

A tube 27, extending through the cap 12 and sealed at its outer end 27a,contains a glass ampoule 28. The ampoule is used to store the cesiumcharge prior to the time the beam source is placed in use. It alsofacilitates charging of the reservoir 16 under the high vacuumconditions which exist in the molecular beam system of which the sourceis a part. More specifically, after the source has been connected to amolecular beam tube and evacuated, the tube 26, which may be of copper,for example, is crushed without fracturing it, thereby breaking theampoule 28 and releasing the contents thereof into the reservoir 16.

The reservoir 16 is a porous mass having a large internal surface areacapable of being wetted by the cesium charge. Thus, when the charge isreleased from the ampoule 28, it rapidly spreads out along this surfaceto form a thin film thereon. I have found that stainless steel woolmakes an excellent reservoir. By way of illustration, the wool may havea fiber size of 1.3 mils, with the fibers compressed to form a densityof 8.3 grams per cubic centimeter. A volume of cubic centimeters willthen hold 0.4 cc. liquid cesium with minimum loss of liquid in a severevibrational environment.

In its normal condition, the surface of the stainless steel woolcontains a number of oxides which are not wetted by cesium. Therefore,the material must be prepared for use by removing these oxides and otherimpurities having a similar effect. The first step is to pass the wallthrough a degreasing process to remove the grease therefrom. Next, it isheated in a hydrogen atmosphere to reduce the oxides. The atmosphereshould have a low moisture content, e.g., a dew point of 60 F. Thetemperature should be over 1600 F. to insure reduction of the chromiumoxides, which are the most 'diflicult to reduce.

The reservoir 16 should maintain firm contact with the inner surface ofthe tube 10 to prevent the liquid cesium from escaping along thissurface. This requirement is consonant with the desirability ofcompressing the steel wool in order to increase the liquid holdingcapacity per unit volume, since the compression forces the wool, whichis fairly elastic, into intimate engagement with the surface Tim. Thecompression is maintained by a retaining screen 30 fastened to a fiatring 32 brazed to the tube 10 adjacent to a shoulder 34.

As pointed out above, the barrier 18 may also be of stainless steelwool. However, to prevent the passage of liquid cesium, it should not bewetted by this material. Accordingly, it is prepared in a mannercalculated to give the opposite results from the treatment accorded thesteel wool of the reservoir 16. More specifically, after degreasing, itis heated in a hydrogen atmosphere saturated with water. This insuresrather complete oxidation of the chromium on the surfaces of the fibers,thereby giving them a non-wettable characteristic with a cesium charge.

The interstices between the fibers in the barrier 18 should be smallenough to prevent liquid flow, which can occur in spite of thenon-wettable characteristics, if the spaces between the fibers aresufiiciently large. With a fiber size of 1.3 mils, the wool may becompacted to a density of 8.3 grams per cubic centimeter to provide aneffective impediment to liquid flow under the vibration al conditionslikely to be encountered in operation. The compression required for thedesired density is provided by a second retaining screen 36 which alsoserves to fix the barrier 18 in its proper position. The screen 36 is 4afiixed to a ring 38 brazed to .the tube 10 adjacent to a shoulder 40.

As seen in FIGURES l and 2, the collimator 20 is disposed between andclamped in position by a pair of supporting members 42 and 44. Themembers 42 and 44 conform to the innersurface of the tube 10 and aresecured against a shoulder 46 within the tube by a retaining ring 48brazed in place. In its preferable form, the collimator comprisesalternate fiat strips 50 and corrugated strips 52 of nickel foil (FIGURE2). The cesium atoms entering the collimator 20 from the chamber 22issue from the opposite end of the collimator as a narrow, well-definedbeam, as required in a molecular beam frequency standard.

The liquid retention characteristics of the molecular beam source arefurther enhanced by the location and operation of the heating coil 24.The temperature of the coil 24 is controlled by a thermostatic switchschematically indicated at 54 (FIGURE 1) which is set to maintain thetemperature of the reservoir 16 at 'C., a level high enough to supplysufficient cesium vapor at the output of the source. The temperature ofthe collimator 20, which is within the coil 24, will then be somewhathotter, say C., than the reservoir 16. Thus, the coolest location whichthe cesium may contact is within the reservoir 16, and the temperatureprogressively increases as one moves from the reservoir to the rightthrough the barrier 18 and the chamber 22 to the collimator 20.

Should some of the liquid cesium escape from the reservoir 16, it willarrive at a warmer portion of the molecular beam source; there it willtend to evaporate faster than the liquid within the reservoir.Furthermore, gases contacting the escaped liquid will tend to condenseat a slower rate than those in contact with the charge contained in thereservoir 16. Consequently, there is a tendency for the escaped cesiumto migrate back into the region of lowest temperaturethe reservoir 16.Thus, over a substantial period of time, there will be essentially nonet flow of liquid from the reservoir 16, even under the severest ofenvironment conditions.

The insulating cover 26 serves to insulate the tube 10 and the partscontained therein from the effect of the ambient temperature and therebymaintain the desired temperature gradient within the source.

The molecular beam apparatus of which the beam source is a component isgenerally internally evacuated to a hard vacuum, preferably 10* mm.mercury or better. Therefore, the source must be effectively sealed fromthe atmosphere. This may conveniently be accomplished by brazing the cap12 to the tube 10, and, accordingly, the material of which these partsare made should be susceptible of joining by suitable brazingtechniques. It should also be relatively inert, particularly withrespect to cesium, and it should emit little or no contaminatingmaterial into the apparatus. Oxygen-free, high-conductivity copper meetsthis requirement, and, additionally, it facilitates conduction of heatfrom the heating coil 24 to the reservoir '16. The members 42 and 44should also be of copper to facilitate conduction of heat inwardly tothe collimator 20.

Copper is wetted by cesium, and therefore liquid cesium introduced tothe reservoir 16, coming in contact with the surface 10a of the tube 10,will tend to spread out along this surface. To prevent the liquid on thesurface 10a from spreading beyond the limits of the reservoir 16, Iprefer to fabricate the ring 32 of a material such as nickel (grade A orbetter), which is not wetted by cesium. The ring 32, which is brazed inplace, thus prevents surface flow of cesium to the right (FIGURE 1)along the inner surface of the tube 10. For the same reason, the ring 38should be of a material not wetted by cesium. The screens 30 and 36 mayalso be fabricated from nickel.

Thus, I have described an improved molecular beam source adapted toprovide a beam of gaseous molecules evolved from a liquid charge. Thecharge is contained in a reservoir having a large surface area ofmaterial wetted by the charge, and, accordingly, the liquid spreads outalong the reservoir surfaces to form a thin film there on whichgenerally adheres even when exposed to the severest inertial effects. Inits preferable form, the reservoir consists of a wool or other fibrousmaterial which permits free flow of the gaseous molecules whileretaining the liquid. The retention characteristics of the material areaided by the fact that, should some of the liquid contained in thereservoir be dislodged, it faces a tortuous labyrinthine path to escapefrom the reservoir, making it extremely likely that it will contact andadhere to some of the many surfaces encountered.

Following the reservoir is a barrier, which again may be of wool, thoughof a material not wetted by the liquid charge. While permitting gas toflow through it, the barrier, whose interstices are considerablyrestricted, serves an effective impediment to the flow of liquid.Accordingly, there is a minimum probability that any of the liquidcharge will escape from the molecular beam source.

It will be apparent that other porous or spongy masses than wool may beused in constructing the reservoir and barrier. For example, anaggregate of small particles may be sintered to form a spongy blockhaving a large internal surface and providing many of the desirable characteristics set forth above.

It will thus be seen that the objects set forth above, among those madeapparent from the preceding description, are efl'iciently attained and,since certain changes may be made in the above construction withoutdeparting from the scope of the invention, it is intended that allmatter contained in the above description or shown in the accompanyingdrawing shall be interpreted as illustrative and not in a limitingsense.

It is also to be understood that the following claims are intended tocover all of the generic and specific features of the invention hereindescribed, and all statements of the scope of the invention which, as amatter of language, might be said to fall therebetween.

I claim:

1. A molecular beam source adapted to form a beam of gaseous moleculesevaporated from a liquid charge, said source including a housing havingan exit aperture, a reservoir containing said liquid charge and disposedwithin said housing, said reservoir comprising an inert porous mass ofliquid adsorbent elements, said elements being elastically compressed bysaid housing, and said mass being wettable by said charge, whereby saidcharge spreads out over the internal surfaces of said elements to form athin fil-m thereon.

2. The combination defined in claim 1 including a collimator disposed insaid aperture to collimate the gaseous molecules evaporated from saidcharge and issuing from said housing.

3. The combination defined in claim 2 including a heating elementadapted to raise the temperature of said charge to facilitateevaporation thereof.

4. The combination defined in claim 1 including a liquid barrierinterposed between said reservoir and said aperture, said barrierproviding a plurality of capillary passageways between said reservoirand said aperture, the surfaces of said passageways being of a materialnot Wettable by said liquid charge.

5. A molecular beam source adapted to form a beam of gaseous cesiummolecules evaporated from a charge of liquid cesium, said sourceincluding a housing having an exit aperture, a reservoir for said liquidcharge disposed within said housing, said reservoir comprising a mass ofdeoxidized steel wool, a charge of liquid cesium in the form of films onthe fibers of said wool, and a collimator disposed to form a beam of thegaseous molecules evaporated from said charge and issuing from saidaperture.

6. The combination defined in claim 5 including a barrier substantiallyimpervious to said liquid cesium disposed between said reservoir andsaid collimator, said barrier comprising a mass of steel wool having amaterial on the surfaces of the fibers thereof which is not wettable byliquid cesium.

7. The combination defined in claim 6 including a. heating elementdisposed around said collimator and adapted to raise the temperature ofsaid charge to facilitate evaporation thereof.

8. A molecular beam source adapted to form a beam of gaseous moleculesevaporated from a liquid charge, said source including a housing havingan exit aperture, a liquid charge, a reservoir for said charge disposedwithin said housing, said reservoir being a sponge whose interiorsurfaces are of a material wettable by said charge, a collimatordisposed to form a beam of gaseous molecules evaporated from said chargeand issuing from said aperture, a barrier disposed in said housingbetween said reservoir and said collimator, said barrier being a porousmass whose inner surfaces are not wettable by said charge, said barrierhaving a plurality of interstices providing passageways therethrough,said interstices being of capillary dimensions, thereby to substantiallyimpede the passage of the liquid of said charge from said reservoir tosaid collimator, and a heater adapted to raise the temperature of saidcharge to facilitate evaporation thereof.

9. The combination defined in claim 8 in which said heater is disposedadjacent to said collimator, whereby said collimator has the highesttemperature in said source and the temperature of said source decreasesbetween said collimator and said reservoir.

References Cited in the file of this patent UNITED STATES PATENTS

